A number of selective hydrogenation reactions are of great technological interest. Olefin chemistry covers a broad field in petrochemistry. As an example, it is technologically important to remove diolefins from thermally cracked gasoline. The selective hydrogenation of butadiene is particularly noteworthy. Further, the selective hydrogention of isoprene or cyclopentadiene should be mentioned. It is very difficult to selectively hydrogenate dienes with isolated double bonds and in particular to control the hydrogenation of higher homologues of these dienes. They normally react during hydrogenation like mono-olefins, that is, they are immediately perhydrogenated to the corresponding alkane. If a selective hydrogenation is to be achieved which stops at the mono-olefin, the reaction should be preceded, according to the dogma, by a double bond isomerization to a conjugated diene. This isomerization is influenced, as a rule, by the surface property of the support material, for instance its acidity.
The most wide-spread and most economical catalyst systems for selective hydrogenation contain as catalytically active substances d-metals of Group VII of the Periodic Table on an inert oxide support material, in particular on aluminum oxide. Selectivity with respect to formation of alkenes from diolefins can be increased by a partial poisoning of the metal component, e.g., the palladium component, with Pb, Zn, Hg, Cd, Sn, etc. (so-called "Lindlar catalysts," cf. H. Lindlar, Helv. Chim. Acta, 35, 446 (1952) or generally G. C. Bond, "Catalysis by Metals," Academic Press, London, New York (1962), 99 and 297). On the other hand, correct dosage in the addition of the catalyst poison is a problem. Often the poisoned catalysts are either unselective or entirely inactive.
There exists further a number of commercial catalysts for selective hydrogenation. These catalysts contain palladium in concentrations of 0.03 to 0.5 wt.% on different aluminum oxides. Also a specific poisoning with Cr has been employed, about 0.5 wt.% Cr having been added. But these catalysts were often insufficiently selective or, if selective, insufficiently active.